(1) Field of the Invention
The present invention relates to a liquid crystal display device, and in particular to a liquid crystal display device having a conductive film on the display side of the liquid crystal display panel.
(2) Description of the Related Art
Liquid crystal display devices are used as display devices for many electronics, such as monitors, cellular phones and portable information terminals. In liquid crystal display devices, a liquid crystal layer is sandwiched between two facing transparent substrates. Electrodes for applying an electrical field to the liquid crystal layer corresponding to the pixels and wires for driving the electrodes are formed on one of the facing transparent substrates and color filters are formed on the other transparent substrate.
In liquid crystal display devices where no electrodes are formed on the transparent substrate on the color filter side, such as in-plane switching (IPS) liquid crystal display devices, a transparent conductive film, such as of ITO, is formed on the rear of the transparent substrate (the surface of the liquid crystal display panel on the display side), so that the transparent conductive film is electrically grounded, in order to prevent the transparent substrate on the color filter side from being charged and generating an unnecessary electrical field. In addition, the polarizing plate that is pasted to the transparent substrate may be conductive, as in JP2006-330230A. When a transparent substrate is charged and an unnecessary electrical field is applied to the liquid crystal layer, a problem may arise, such that some of the pixels do not display the correct color, or the entire screen turns up whitish, and thus the display does not function normally.
FIG. 1 is a plan diagram showing a liquid crystal display device as viewed from the front, and FIG. 2 is a cross sectional diagram along the line A-A′ in FIG. 1. As shown in FIG. 2, the liquid crystal display device has a liquid crystal display panel LCP where a liquid crystal layer is sandwiched between two facing transparent substrates that are held in a predetermined position by a mold frame MFR made of a resin. The mold frame MFR is contained in a lower frame SFR, and a backlight, not shown, is provided between the liquid crystal display panel LCP and the lower frame SFR.
An upper frame UFR is provided around the periphery of the liquid crystal display panel LCP. The upper frame UFR is provided so as to cover the lower frame SFR and secured to the lower frame SFR by means of a fixing means, such as screws.
The lower frame SFR and the upper frame UFR are usually formed of a metal, such as steel. In order to ground the conductive film (transparent conductive film or conductive polarizing plate) formed on the liquid crystal display panel on the display side, as described above, a conductive rubber CG may connect the conductive film of the liquid crystal display panel LCP and the mold frame MFR as shown in FIG. 2. In addition, the conductive rubber CG is made to make contact with the upper frame UFR, and furthermore the upper frame UFR and the lower frame SFR are combined, so that the conductive film is electrically grounded to the lower frame.
The conductive rubber CG is thicker than the clearance between the mold frame MFR and the upper frame UFR. In addition, as shown in FIG. 3, when the upper frame UFR is screwed to the lower frame SFR at the point B, the upper frame is pushed in the direction of the arrow, and as a result part of the upper frame UFR on the display side becomes easy to lift in the direction of the arrow C.
Therefore the electrical contact between the conductive rubber CG and the upper frame UFR becomes insufficient, causing the surface of the liquid crystal display panel to become charged and causing defects in the display. In addition, in the case where the upper frame UFR is made of aluminum in order to reduce the weight of the liquid crystal display device, the upper frame UFR lifts more. In addition, when the surface of aluminum is oxidized, sufficient electrical connection with the conductive rubber becomes more difficult to secure.
In addition, JP2006-330230A discloses a technology for releasing a charge in the panel by pasting a conductive sheet to the corner portion of the panel with the sheet extending to the lower frame, and thus grounding the conductive polarizing plate provided on the liquid crystal display panel. However, the conductive sheet can be pasted only in the corner portion, where supports for the liquid crystal display panel are provided so that the cell can be secured, and therefore there is much less freedom of design, for example for the form of the mold frame and the arrangement of the conductive sheet. In addition, the supports of the liquid crystal display panel being lower than the upper surface of the conductive sheet that is pasted on the panel easily causes the brightness to be inconsistent, and therefore setting of the clearance between the mold frame and the upper frame becomes very difficult.
In the case where the conduction from the conductive film to the lower frame in the liquid crystal display panel is secured only with conductive rubber, it is necessary for the clearance between the upper frame and the lower frame to secure a certain thickness for the conductive rubber, and therefore it becomes difficult to make the frame of the liquid crystal display device narrower. In addition, there is a risk that the conductive rubber may be compressed or become disconnected when the mold frame and the lower frame are covered with the upper frame.